[Python] Python code coverage not working for multiprocessing inside site-packages

I have a simple code packages in a module/folder "src" File sample.py

import multiprocessing

def f(x):
return x*x

def big_run(n):
with multiprocessing.Pool(5) as p:
p.map(f, list(range(n)))
p.close()
p.join()



File __init__.py

from .sample import big_run



And I test it with a file test.py

from src import big_run

def test_main():
big_run(10)



Just copy the folder to /mnt/data/global_envs/seb_py310/lib/python3.10/site-packages I use pytest with pytest.ini

[pytest]
python_files = test*.py
required_plugins = pytest-cov pytest-xdist pytest-env

[coverage:run]
concurrency = multiprocessing
branch = true
parallel = true
sigterm = true



And the command line python -m pytest . --cov=src --cov-branch --cov-report=term --cov-config=../../../pytest.ini

The code coverage report is

---------- coverage: platform linux, python 3.10.6-final-0 -----------
Name Stmts Miss Branch BrPart Cover
----------------------------------------------------------------------------------------------------------------
/mnt/data/global_envs/seb_py310/lib/python3.10/site-packages/src/__init__.py 1 0 0 0 100%
/mnt/data/global_envs/seb_py310/lib/python3.10/site-packages/src/sample.py 8 1 2 0 90%
----------------------------------------------------------------------------------------------------------------
TOTAL 9 1 2 0 91%


And if I just put the folder next to the test.py (not inside site-packages) I get 100% code coverage. The code that is not covered is the line return x*x. If I don't use multiprocessing, obviously, there is no issue.

Any idea why I can't get 100% code coverage when it is inside site-packages? I care about this because I am building a python library that uses multiprocessing.pool and I can't get the right coverage level.

I tried moving the code in many other folders, the issue is only with site-packages. I also tried copying the whole content of site-packages to another folder such as /mnt/data/global_envs/seb_py310/lib/python3.10/sites and I still get 100% code coverage. I also looked into other settings for pytest.ini and using plain coverage but nothing worked. I also tried this https://stackoverflow.com/a/41365659/11598039

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